Fossils of a previously undiscovered species of dinosaur have been found in slabs of Utah sandstone that were so hard that explosives had to be used to free some of the remains, scientists said Tuesday. The bones found at Dinosaur National Monument belonged to a type of sauropod — long-necked plant-eaters that were said to be the largest animal ever to roam land.

The discovery included two complete skulls from other types of sauropods — an extremely rare find, scientists said.The fossils offer fresh insight into lives of dinosaurs some 105 million years ago, including the evolution of sauropod teeth, which reveal eating habits and other information, said Dan Chure, a paleontologist at the monument that straddles the Utah-Colorado border.

“You can hardly overstate the significance of these fossils,” he said.Of the 120 or so known species of sauropods, complete skulls have been found for just eight. That’s mostly because their skulls were made of thin, fragile bones bound by soft tissue that were easily destroyed after death.More